1. Field of the Invention
This invention relates to a laser hole boring (drilling) apparatus for printed circuit boards or packages. The printed circuit board of electronic apparatuses is equipped with tiny electronics devices, e.g., ICs, LSIs, capacitors, resistors or so. For mounting the electronic devices, plenty of pin holes are bored through the printed circuit board. At present, the holes of the printed circuit board are bored one by one by a mechanical drilling apparatus which repeats the operation of moving a rotating microdrill to a determined spot, lowering the microdrill onto the spot of the object printed circuit board, boring a hole and raising the microdrill.
The mechanical boring is endowed with flexibility. The mechanical processing is preferable for the purpose of perforating holes of a small number of products having various dispositions of holes. The prevalent boring method is still the mechanical boring. The mechanical processing has a drawback of low drilling speed, because the microdrill bores holes one by one.
This application claims the priority of Japanese Patent Application No. 11-177376 (177376/1999) filed Jun. 23, 1999 and No.2000-154514 (154514/2000) filed May 25, 2000 which are incorporated herein by reference.
2. Description of Related Art
In the description, the word xe2x80x9crayxe2x80x9d means an individual line representing an optical path of light. A beam is an assembly of rays which starts from a point, travels along different paths and converges at another point. The beam is a collective concept. The ray is an individual concept. But a beam can be drawn by a line. The light packets which have different starting points or different converging points cannot be deemed as a beam. The beam should be discriminated from the ray. Raising mounted device density and multilayered wiring of recent printed circuit boards requires to bore smaller diameter holes. Packages for semiconductor devices require plenty of microholes boring. For example, less than 0.15 mm of a hole diameter will be required in near future. It is difficult to bore such a small hole by the mechanical drilling. The small hole diameter requires a smaller drill diameter. Such a small diameter deprives the drill of the strength of maintaining itself as a tool. A promising candidate of perforating such small holes is an optical boring by laser beams. This invention aims at providing a high speed laser boring apparatus which is preferable f or mass production of a small number of types of printed circuit boards.
A laser beam scanning boring method using a laser, galvanomirrors and lenses has been proposed as a new boring method which will be faster than the present mechanical boring method. This method uses a pulse laser beam of a high power laser, for example, a CO2 laser. The laser beam scanning boring apparatus perforates holes one by one by the steps of swaying galvanomirrors by a unit angle, reflecting a pulse laser beam by the galvanomirrors, converging the beam on a point of a printed circuit board by a lens and burning out a hole through the board by heat in an instant. This method scans on an object printed circuit board with a pulse laser beam by deflecting the laser beam in two directions perpendicular with each other by two galvanomirrors which sway in definite amplitudes with predetermined speeds around different axes vertical to each other. Since the laser generates a pulsed beam, the pulsed laser beam perforates small holes at discrete, predetermined spots despite the continual oscillation of the galvanomirrors. The laser beam scanning boring method can perforate smaller holes at a faster rate than the mechanical drilling, since a small pulse beam converged by the lens burns small areas of the board. The laser beam scanning method is a promising, novel boring method which will be realized in near future. However, the Inventor of the present invention is aware that this novel laser beam scanning method would have some problems.
A first problem is the converging lens. A conventional and traditional lens has an action of converging rays of a beam together on a spot of an image plane distanced by a focal length from the lens. A perpendicularly incident beam is converged onto the center of the image plane. Another beam slanting at an angle xcex8 to the optical axis is converged on a spot distanced by f tan xcex8 from the center of the image plane. The distance of an image spot from the center on the image plane is here defined as a xe2x80x9cheightxe2x80x9d of the image spot. Since the height of the spot image of a xcex8 inclining beam is f tan xcex8, the conventional, traditional lens can be called here an xe2x80x9cf tan xcex8xe2x80x9d lens.
When the galvanomirrors scan a single laser beam in the x-direction and in the y-direction at a constant rate, the velocity of the swaying angle xcex8 is constant. The interval between the neighboring holes on the board should be constant. The conventional f tan xcex8 lens cannot satisfy such a linear relation h=fxcex8 between the angle xcex8 and the spot height h. The beam of an incidence angle xcex8 should make an image at a spot of a height of fxcex8 by a lens for boring holes at a constant interval. Instead of conventional traditional lenses, the laser beam scanning method requires a special lens which makes xcex8 inclining rays converged on a spot image of an fxcex8 height (h=fxcex8) on the image plane. Such a lens is called an xe2x80x9cfxcex8xe2x80x9d (f-theta) lens. The fxcex8 lens is different from the conventional lenses. The fxcex8 lens which is realized by an assembly of plurality of lenses requires a special design for giving the fxcex8 property.
Another problem of the laser beam scanning method is the orthogonality of the beam to the board. The scanned beam should always be vertical to the board for boring holes perpendicular to the board. Although the incident beams are slanting to the lens axis and pass the lens at non-central point, the beams must construct a outgoing, convergent beam perpendicular to the board. This is a very difficult property for a conventional lens to realize. The property that slantingly-incident beams are converted into a vertical converging beam by a lens is called xe2x80x9ctelecentricityxe2x80x9d or xe2x80x9ctelecentric propertyxe2x80x9d. The laser beam scanning boring method by the galvanomirrors requires the xe2x80x9cfxcex8 propertyxe2x80x9d and the xe2x80x9ctelecentricityxe2x80x9d for the converging lens.
It is expected that the galvanomirror laser beam scanning boring method by the galvanomirrors and the fxcex8 lens would enable the hole boring processing on printed circuit boards or on IC packages to perforate up to 500 holes per a second. FIG. 1 shows a schematic view of the laser beam scanning boring apparatus which will be applied to an actual processing in near future. A laser beam 1 is once reflected by an x-axis scanning galvanomirror 2. The laser beam is again reflected by a y-axis scanning galvanomirror 3. The x-axis scanning galvanomirror 2 sways along the x-axis. The reflected beam sways right and left in the x-direction in a definite amplitude with determined timing. The y-axis scanning galvanomirror 3 sways along the y-axis. The reflected beam sways up and down in the y-direction in a definite amplitude with determined timing. The timing of moving in the x- and y-directions depends upon the mode of the scanning.
The twice reflected beam scans horizontally and vertically on a lens 4. The lens 4 converges the beam on a printed circuit board 5. Since the beam runs both in the x-direction and in the y-direction and the laser emits a pulsed beam, the apparatus can perforate a lot of holes 6 aligning their locations in the x-direction and in the y-direction at an ultrahigh speed. The laser beam scanning boring method will make big progress over the current mechanical drilling in speed in near future. Practical models have been produced for galvanomirrors of small inertia and galvanometers for oscillating the mirror at a high speed. The far-infrared lenses have been designed and prepared for CO2 lasers.
However, a request for perforating holes at a higher speed than the laser beam scanning boring method is suggested. The upper limit of the laser beam scanning boring method is thought to be 500 holes per second at most. The laser beam scanning method cannot perforate more than one hole simultaneously, since the method uses a single beam essentially. The request is to perforate several thousands of holes per second which could be satisfied neither by the current mechanical drilling nor by the current proposed laser beam scanning boring method. The laser beam scanning boring method which scans a laser beam by oscillating mechanically the galvanomirrors will be unable to catch up with the request of an increasing number of holes bored per second.
A DOE (Diffractive Optical Element) is proposed for an optical element for dividing a single laser beam into many beams having desired spatial (angular) distribution by utilizing the diffraction phenomenon. Unlike the beam scanning boring method by the galvanomirrors, the DOE method uses many beams for perforating many holes simultaneously. The DOE is explained by referring to FIGS. 3, 4 and 5. FIG. 3 is a plan view of a DOE. FIG. 4 is a plan view of a unit pattern. FIG. 5 is an Lxe2x80x94L section of FIG. 4. A DOE 8 is a plate transparent to the object light. The DOE is divided into plenty of unit square or rectangular patterns aligning crosswise and lengthwise. Every unit pattern 9 is further divided into many pixels 10 by horizontal lines and vertical lines. The thicknesses of the pixels are diversified into 2m (m=1,2,3, . . . ) steps with a primary unit of xcex/(nxe2x88x921)2m. The diversification aims at dividing a thickness equivalent to a single wavelength optical path into 2m steps. Then, the distribution of the thickness corresponds to the phase distribution of DOE, which modulates the phase of the incident light passing through the DOE.
FIG. 5 shows the simplest example of binary step type (m=1) which diversifies the thickness into binary steps in which the phase difference is 180 degrees, a half of the wavelength. There are only two different heights of steps. The unit pattern 9 has higher (thicker) pixels 11 and lower (thinner) pixels 12. The thicker pixel induces a half wavelength phase delay from the thinner pixels. The thickness distribution determines the diffraction pattern being made by the DOE on the image plane. The distribution of the pixels diffracts the laser beam into the whole directions for making a desired spot array pattern on the image plane.
All the unit patterns 9 have the same thickness distribution of pixels. Namely, the DOE is an assembly of the same unit patterns aligning in two dimensions. Thus, the scope of designing is restricted within a unit pattern. The length of the side of the unit pattern 9 is the period xcex9 of the phase distribution of the DOE. Repetition of the same patterns plays an important role in diffraction phenomena, as described later in detail. The spatial period xcex9 determines the divergence of the diffraction angles. A smaller period xcex9 corresponds to bigger diffraction angles. A bigger spatial period xcex9 corresponds to smaller diffraction angles.
The unit pattern restricts the scope of the phase (thickness) variables. Namely, the variables are the distribution of phase in a unit pattern of the DOE. Two dimensionally diverging diffraction beams are made by giving two dimensional distribution of the phase differences to the pixels in a unit pattern. A square unit pattern (xcex9xc3x97xcex9) can equalize the unit diffraction angles for both the x- and y-directions. Otherwise, a rectangle unit pattern (xcex9xxc3x97xcex9y) can differ the unit diffraction angles for the x-direction and y-direction. An arbitrary diffraction beam distribution is realized by determining the size of a pixel and the thickness (phase) distribution of pixels. The DOE can make an arbitrary number M of beams from a single beam by diffraction unlike the single beam of the galvanomirror method. The DOE is not moved unlike the galvanomirrors. The DOE is static. Despite the static feature, the DOE can simultaneously perforate many holes by many divided beams. The many divided beams are converged by a lens and irradiate on an object printed circuit board or an object device package.
A new problem appears with regard to the lens for converging the multiple beams produced by the DOE. An conventional traditional lens is an f tan xcex8 lens. It has been explained that the galvanomirror beam scanning boring apparatus should employ not an conventional f tan xcex8 lens but an fxcex8 lens. The galvanomirror has the least unit xcex94 of a swaying angle for a pulse interval. The swaying angle at the instant of pulsation should be a product mxcex94 of an integer m and the least unit xcex94. The fxcex8 lens is the best lens for the galvanomirror scanning boring apparatus for boring holes at a constant interval.
An fxcex8 lens is composed mainly of five lenses or four lenses for satisfying the fxcex8 c property, the telecentricity and the diffraction limited convergence. At least two lenses are required for building an fxcex8 lens. FIG. 2 shows a simple example of an fxcex8 lens having two lenses including a first lens L1 and a second lens L2. The first lens L1 has a front surface S1 on the object side and a rear surface S2 on the image side. The second lens L2 has a front surface S3 on the object side and a rear surface S4 on the image side, The first lens L1 is a spherical lens. The second lens L2 has a spherical front surface S3 and an aspherical rear surface S4 in this example. The sign (plus or minus) of a curvature of a lens surface is defined to be positive for the center of the curvature lying on the image side and to be negative for the center lying on the object side. The aspherical surface is expressed by the lens surface height (sag) Z(r) as a function of the radius r. Eq.(1) is the definition of the height Z(r), where c, k and xcex1j are parameters. The number j increases by one from 1 to s (integer). The upper limit number s is determined for obtaining the expected performance. In the example, the upper limit is s=5.                               Z          ⁡                      (            r            )                          =                                            cr              2                                      1              +                                                {                                      1                    -                                                                  (                                                  1                          +                          k                                                )                                            ⁢                                              c                        2                                            ⁢                                              r                        2                                                                              }                                                  1                  2                                                              +                                    ∑                              j                =                1                            s                        ⁢                          xe2x80x83                        ⁢                                          α                j                            ⁢                                                r                                      2                    ⁢                    j                                                  .                                                                        (        1        )            
Table 1 denotes the data (lens number; surface number; curvature radius; thickness, spacing; and refractive index) of the example fxcex8 lens.
The thickness, spacing means a center thickness of the lens, a center distance between the lenses, or a center distance between the lens and the image. For example, 65.114 mm is the distance between S2 of L1 and S3 of L2. S1, S2 and S3 are spherical. S4 is aspherical 158.588 mm is the distance between S4 of L2 and the image plane I. The light source is the CO2 laser emitting coherent light of a 10.6 xcexcm wavelength. The lens is made of ZnSe which is transparent for the infrared (10.6 xcexcm) light. The refractive index is defined to the far infrared light.
Table 2 shows the data for the aspherical surface S4.
If the fxcex8 lens were adopted into the DOE apparatus, multiple beams divided into by the DOE would be individually converged on the image plane by the fxcex8 lens. The spot positions are determined by the slanting (diffraction) angles of the individual beams. However, the Inventor is aware that the fxcex8 lens is not the best choice for the DOE.
The reason why the fxcex8 is not the best choice is explained as follows. The DOE defines pixels in definite spatial periods for making diffraction beams by modulating the phase of the passing light by the variation and the distribution of step heights (thicknesses). Instead of random distribution of thickness, the DOE consists of the equivalent patterns having regularly arranged pixels. Thus, restrictions are imposed upon the lens.
A parallel beam is assumed to enter the DOE at right angles. In the case of Fraunhofer type DOE, each of the divided beams is still a parallel beam, although all the diffracted beams are no longer parallel to the optical axis. The parallel beams do not converge without a lens. Thus, the DOE requires a lens for converging different diffraction order beams on an image plane at a rear focal point. Since all the diffraction beams consist of parallel rays, all order diffraction rays converge on the same plane by the lens. Namely, the DOE divides a single laser beam into divided diffraction beams and the lens converges all the divided beams into discrete spots on the image plane I.
The incompetence of the fxcex8 lens will be explained more in detail. xcex is a wavelength of the laser light. xcex9 is a spatial period of the unit pattern. The order of diffraction is denoted by xe2x80x9cnxe2x80x9d. The n-th order diffraction beam is inclined at xcex8n n to the optical axis. Namely, xcex8n is the diffraction angle of the n-th order beam. The equation for diffraction is denoted by
xcex8n=nxcex.xe2x80x83xe2x80x83(2)
The inclination angle xcex8n ( of the n-th order diffraction beam is determined by
xcex8n=sinxe2x88x921(nxcex/xcex9).xe2x80x83xe2x80x83(3)
The n-th order beam goes into the lens at the inclination angle xcex8n. The height hn of the n-th order diffraction beam depends on the kind of the lens.
(1) in the case of a conventional f tan xcex8 lens
hn=f tan{sinxe2x88x921(nxcex/xcex9)}xe2x80x83xe2x80x83(4)
(2) in the case of an fxcex8 lens that is the same one applied to the galvanomirror scanning boring apparatus
hn=f sinxe2x88x921(nxcex/xcex9)xe2x80x83xe2x80x83(5)
In any case, the image height hn on the image plane is not in proportion to the diffraction order n. The relation between n and xcex8n deviates from the linear relation.
tan (sinxe2x88x921x)=x+(x3/2)+(3x5/8)+xe2x80x83xe2x80x83(6)
sinxe2x88x921x=x+(x3/6)+(3x5/40)+xe2x80x83xe2x80x83(7)
The deviation from the linearity appears in the terms higher than the third order of x. In Eq.(6) and Eq.(7), x=nxcex/xcex9. If xcex9 is large and n is small, the deviation from the linearity may be negligible. On the contrary, if xcex9 is small and n is large (higher order diffraction), the deviation from the linearity cannot be neglected.
Comparison of two lenses teaches us that the deviation from the linearity is smaller for the fxcex8 lens than the f tan xcex8 lens. The fxcex8 lens is better than the f tan xcex8 lens in the viewpoint of the linearity. The fxcex8 lens is, however, not the best lens for the DOE system yet. It is not the best choice of diverting the fxcex8 lens suitable for the galvanomirror beam scanning method to the DOE system. An ideal lens should be a lens giving linearity to the relation between sin xcex8n and the hn. The lens is novel itself. There is neither a concept nor a name. Thus, a name should be given to the new lens. Such a lens may be called an xe2x80x9cf sin xcex8 lensxe2x80x9d. If such an f sin xcex8 lens were to be made, the lens would realize the following convenient relation.
(3) Assuming that the new f sin xcex8 lens which is proposed first by the present invention would be positioned behind the DOE, the height of the n-th order diffraction beam is given by,                               h          n                =                  f          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      {                                          sin                                  -                  1                                            ⁡                              (                                  n                  ⁢                                      xe2x80x83                                    ⁢                                      λ                    /                    Λ                                                  )                                      }                                              (        8        )                                          xe2x80x83                ⁢                  =                      fn            ⁢                          xe2x80x83                        ⁢                          λ              /                              Λ                .                                                                        (        9        )            
If the f sin xcex8 lens could be produced, the f sin xcex8 lens would be the best lens for the DOE system. If the f sin xcex8 lens were produced and used for the DOE, the interval dh=hnxe2x88x92hnxe2x88x921 between the nearest neighboring diffraction spots hn and hnxe2x88x921 should be constant irrespective of the diffraction order n.
dn=fxcex/xcex9.xe2x80x83xe2x80x83(10)
Three different kinds of lenses have been compared with regard to the linearity between n and hn of the diffraction pattern on the image plane. If only the paraxial beams were dealt with, an f tan xcex8 lens, an fxcex8 lens and an f sin xcex8 lens would make little difference. If xcex8 is small enough, approximate equations hold between sin xcex8, tan xcex8 and xcex8 within a small error,
sin xcex8n=tan xcex8n=xcex8n.xe2x80x83xe2x80x83(11)
For example, if the DOE diffraction system uses a CO2 laser of xcex=10.6 xcexcm as a light source, a lens of a focal length f=127 mm and a unit pattern of xcex9=2.688 mm, the interval is d=0.5 mm. In this case, even the seventh order diffraction beam has a small diffraction angle xcex87=1.6 degrees. The paraxial approximation is valid for the case. The conventional f tan xcex8 lens is applicable to the case of a small diffraction angle. However, if the diffraction angle is large, the f tan xcex8 would cause too large errors and even the fxcex8 lens would bring about big errors.
One purpose of the present invention is to provide a laser hole boring apparatus for boring many holes at a high speed by the DOE. Another purpose of the present invention is to provide a laser hole boring apparatus for boring a lot of holes widely-spread within a large processing Area, utilizing large diffraction angles of higher order diffraction of the DOE. A further purpose of the present invention is to provide a laser hole boring apparatus for boring many holes with the DOE and the f sin xcex8 lens for suppressing the errors of higher order diffraction beams.
The laser hole boring apparatus of the present invention comprises a laser, a diffractive optical element (DOE) having on its surface a microstructure, which is formed by the repetition of similar patterns with a definite spatial period, or by the modulation of the repeated patterns for diffracting the laser beam into a plurality of diffraction beams, and an f sin xcex8 lens for converging the divided beams on an image plane.
Namely, the new laser beam diffraction apparatus has a large power laser, a DOE for making divided multibeams by diffracting the laser beam and an f sin xcex8 lens of converging each divided beam at a determined spot for boring many holes simultaneously.
FIG. 11 shows a schematic view of the laser hole boring apparatus of the present invention. A wide parallel beam 1 is made by a beam expander from a beam emitted from a laser. The wide beam shoots the DOE 8. The DOE 8 makes many diffraction beams 13 with different diffraction angles of two dimensional distribution. Each beam 13 consists of parallel rays. The parallel rays 13 are converted into converging rays 17 by a lens 14. The assembly of the converging rays is called a converging beam. A package 15 is placed at the focus of the lens. The package 15 is an image plane. The many converging beams 17 burn out small holes 16 at the converging spots on the package 15. The beam inclining at xcex8 converges at a spot of a height f sin xcex8. The DOE 8 enables the apparatus to bore many holes simultaneously by making plenty of diffraction beams.
It is important to adopt an f sin xcex8 lens as a convergence lens. The f sin xcex8 lens is a novel lens, which is first proposed by the Inventor, for converging rays going at an angle xcex8 at a height of h=f sin xcex8. The f sin xcex8 lens is different from the conventional f tan xcex8 lens or from the galvanomirror-preferable fxcex8 lens.
Diffraction enables a DOE to convert a single beam into many beams directing in different directions. It is a convenient property of the DOE. The DOE is superior in the yield of boring many small holes to the mechanical drilling or the galvanomirror scanning apparatus. However, the DOE is a permanent device having a definite distribution of the thickness. The DOE makes a definite diffraction beam distribution which cannot be changed in any case. The DOE lacks flexibility. This is a drawback of the DOE boring method. The DOE cannot satisfy the requirement of boring holes of different distributions.
The rigidity is the drawback of the DOE multibeam boring method. The sturdy rigidity prevents the DOE method from being an alternate of the prevalent mechanical drilling. Flexibility should be given to the DOE boring method for employing the DOE boring method as a practical apparatus for boring holes on printed circuit boards or packages. The Inventor has an idea of an improvement for changing the DOEs easily. The improvement can change the hole distribution patterns easily by preparing a rotatable disc having a plurality of windows in the angular direction, making different DOEs, mounting the different DOEs in the windows and rotating the disc for choosing an optimum DOE. When k kinds of DOEs are put on m windows (m greater than k), the DOE apparatus can perforate k different patterns of holes on an object board or an object package at ultrahigh speed.
For compensating the DOE apparatus by endowing further flexibility, the Inventor thinks of hybridizing the DOE apparatus with the galvanomirror scanning apparatus. The galvanomirror scanning method can perforate holes at faster speed than the current prevalent mechanical drilling. The galvanomirror is rich in freedom and flexibility for perforating various distributions of holes. High degree of freedom enables the galvanomirror method to change the speed, the amplitude, the timing and the pulsation interval of the scanning. Free choice of the variables allows the galvanomirror method to make arbitrary distribution patterns of holes. The galvanomirror can respond the change of the size of the board or the change of the interval of holes easily. The DOE is inferior in flexibility to the galvanomirror method but superior to the galvanomirror method in speed. The hybridization of the galvanomirror and the DOE would be able to obtain high speed and rich flexibility applicable to any distribution patterns of holes.
The hybrid-type of the laser hole boring apparatus of the present invention comprises a laser, at least two galvanomirrors swaying in different directions, a diffractive optical element (DOE) having on its surface a microstructure formed by the repetition of similar patterns with a definite spatial period, or by the modulation of the repeated patterns for diffracting the laser beam into a plurality of diffraction beams, an f sin xcex8 lens for converging the divided beams on an image plane, and a device for inserting the DOE into the path of the laser beam or taking off the DOE from the path of the laser beam. Both the galvanomirror scanning and the DOE diffraction are included in the hole boring apparatus of the present invention. When the galvanomirror scanning is adopted, the DOE should be removed from the optical path of the laser beam. When the DOE is employed, the galvanomirror should be stopped in the neutral, rest position.
The hybrid-type boring apparatus of the present invention allows the DOE to intervene in any part of the beam path. For example, an isolated single DOE is optionally put into or got rid of from the middle point between two galvanomirrors.
Otherwise, another apparatus will be realized by a rotatable disc with a plurality of windows having different DOEs interposed in the beam line. Different DOEs can be replaced by rotating the disc.
Alternatively, a further apparatus will be accomplished by a rotatable disc having a plurality of windows having different DOEs and an open aperture interposed in the beam line. Different DOEs and galvanomirror system can be alternatively changed. FIG. 12 denotes the apparatus having the rotating disc of the DOEs and the galvanomirror system. A parallel laser beams is emitted by a CO2 laser (not shown in the figure). The laser beam 1 is reflected and deflected by an x-scanning galvanomirror 2. Next, the x-deflected beam 18 is reflected and deflected by a y-scanning galvanomirror 3. A rotatable disc 19 is furnished in the intermediate beam 18 between the galvanomirrors 2 and 3. The rotatable disc 19 has a plurality of round windows 20 in the angular direction. Several DOEs 8a, 8b and 8c are fitted in the windows 20. Some windows are left open without DOE. The open windows are provided for making use of the galvanomirror scanning system. When the DOE 8 is used, the galvanomirror is at rest at the middle, neutral position.
Furthermore, the size of the hole arrangement patterns can be enlarged or shrunk by installing a device for adjusting the divergence angle of the laser beam between the DOEs and the laser, and also by displacing the DOE toward the laser or toward the lens. The adjusting device for controlling the divergence angle of the laser beam is a set of lenses or a beam expander. The divergence angle of the beam can be raised by a lens having a long negative focal length. Otherwise, a lens having a positive focal length can decrease the divergence angle and can further make a converging beam. Since a beam expander includes at least two lenses, the alteration of the interval between the lenses changes the divergence angle of the output laser beam.
The purpose of this invention is to take advantages of the flexibility of the galvanomirrors and the high performance of the diffractive optical elements. However, a blunt combination of the galvanomirrors and the diffractive optical elements is useless, because the galvanomirror and the DOE require different kinds of converging lenses. As mentioned before, galvanomirrors are suitable to make a deflection device moving at a constant angular velocity. The fxcex8 lens is preferable for the galvanomirror which aims at boring a plurality of holes at a constant interval. On the contrary, the DOE requires an f sin xcex8 lens as a suitable converging lens. The most preferable lens is different for the galvanomirror and the DOE. Unfortunately, there is no lens which is appropriate to both the galvanomirror and the DOE.
Thus, this invention abandons the fxcex8 lens but adopts the f sin xcex8 lens for a common convergence lens. The diffractive optical element (DOE) has poor flexibility but the galvanomirror is endowed with easily controllable parameters. Thus, this invention chooses the f sin xcex8 lens for the sake of the rigidity of the DOE. If the angular velocity of the swaying motion were constant, the fxcex8 lens would be suitable for the galvanomirrors. It is possible to perturb the rate of angular velocity in a ratio of cos xcex8 in the galvanomirror instead of the constant angular velocity. The modulation of the rate of con xcex8 can be realized by mixing higher order harmonics with the fundamental sine wave. The f sin xcex8 lens would enable the galvanomirror to perforate many microholes at a constant interval by accelerating the swaying motion near xcex8=0 and decelerating the swaying motion for bigger xcex8s. When the amplitude of the swaying motion is small, a little addition of harmonics is enough for boring holes at the constant interval, since the difference between f sin xcex8 and fxcex8 is small.
When a definite location of the image plane should be aimed, the galvanomirror should sway in a larger amplitude than the fxcex8 lens. The f sin xcex8 lens needs a bigger swaying angle of xcex1xe2x80x2=sinxe2x88x921xcex1 than fxcex8 lens, a is defined as the swaying angle of the galvanomirror. It is, thus, possible to realize high precision scanning even with the f sin xcex8 lens by controlling the swaying angle or the angular velocity of the galvanomirrors.
DOEs described hitherto mean the Fraunhofer type DOEs which have only the function of dividing a beam into a plurality of beams. This invention, however, can be applied to the Fresnel type DOEs which have finite focal lengths. The advantages of the Fresnel type DOE are annihilation of the 0th order light and high freedom of size-change of hole patterns.
This invention includes a laser, a DOE and an f sin xcex8 lens. The DOE diffracts the laser beam into a plurality of beams with different slanting angles. The f sin xcex8 lens makes spots with a definite interval on an image plane by converging the parallel beams into small spots. The DOE enables this invention to bore many holes simultaneously on printed circuit boards at a high speed. The f sin xcex8 lens satisfies the required location accuracy of the diffraction spots and allows this invention to make use of high order diffraction beams. The use of high order diffraction beams raises the number of holes which can be bored at the same time.
Furthermore, this invention assembles the galvanomirror beam scanning system with the DOE diffraction system. According to the hole arrangement patterns, either the galvanomirror beam scanning system or the DOE diffraction system is chosen for boring multiholes on printed circuit boards. The high speed boring of the DOE system and the high flexibility of the galvanomirror scanning system realize an excellent hole boring apparatus by compensating the counterparts for the defects. If the DOE is positioned at the middle of two galvanomirrors, the telecentricity of the f sin xcex8 lens exhibits the highest performance.
A rotatable disc having windows with a plurality of different DOEs enables the system to change various hole pattern processing in an instant by rotating the disc. The rotary disc is useful in the hybrid apparatus including the DOE boring system and the galvanomirror boring system. In the hybrid case of the Fraunhofer DOE and the galvanomirror, at least one window should be left to be an open window. The open window allows the galvanomirror system to scan the pulse laser beam on the object print board. In the hybrid case of the Fresnel DOE and the galvanomirror, at least one window should be installed with a lens having the same focal length as the Fresnel DOE. The lens-furnished window allows the galvanomirror system to scan the pulse laser beam on the object print board placed at the same height as the case of the Fresnel DOE. The rotary disc facilitates to exchange the DOE system reciprocally with the galvanomirror system.
The hole interval can be raised or reduced by adjusting the dispersion angle of the incident laser beam by, e.g., a beam expander or a lens and by displacing the DOE forward or backward. The DOE-displacing adjustment enables the system to change the hole interval. Another controlling of the hole spacing is based on the mask imaging optical systems. The size of the spot image is determined by the size of the pinhole. The hole interval can be increased or decreased by changing the position of the pinhole mask or by displacing the DOE.
This invention can make use of both the Fraunhofer type DOE having an infinitely long focus and the Fresnel type DOE having a finite focus. The change of the distance between the lens and the DOE enables the present invention to change the interval of the diffraction patterns. The use of the Fresnel DOE enlarges the scope of the size change of the patterns. The Fresnel DOE can eliminate the influence of the 0th order noise light.